Display devices, such as liquid crystal displays (LCDs) or organic light emitting displays (OLEDs), are used in a variety of environments. Depending on the use environment and application of the display device, brightness and contrast features of the display device may be of significant value. For example, use of a display device in an outdoor environment or in an environment with high ambient light may require increased levels of contrast and/or brightness to maintain a desired level of viewability.
Typical LCDs comprise a liquid crystal display panel having a thin film of liquid crystals sandwiched between a pair of transparent electrodes. The liquid crystal display panel typically includes a pair of glass plates, the glass plates being sealed together around their respective edges. The glass plates may be assembled with spacers between them to maintain a constant separation distance. Two crossed axis polarizers may be adhered to the respective inside surfaces of the glass plates, one polarizer being adhered to the front of the liquid crystal display panel and the other polarizer being adhered to the rear of the liquid crystal display panel. When a voltage is applied selectively across the electrodes, the liquid crystal molecules between them may be rearranged or switched in polarization so that light is either transmitted or absorbed in the output polarizer to form characters or graphics.
LCDs may include a layer of indium tin oxide or another suitable material positioned between, or in front of, the front polarizer and the front glass plate for use as an electromagnetic interference shield. Some LCDs, intended for use in cold environments, also include a layer of indium tin oxide or another suitable material positioned between the rear polarizer and the rear glass plate and electrically connected to a power source for use as a heating element.
LCDs may be susceptible to back-reflected ambient light such that the viewing characteristics of the display deteriorate under high ambient light conditions, such as when placed in direct sunlight. Various methods have been used to improve the viewing characteristics of the LCDs. For example, in some embodiments, anti-reflection coatings have been applied to the front of the display. As another example, additional optical layering has been used to improve the viewing characteristics and to improve the durability of the displays.
It is known to add a transparent layer, also referred to herein as an overlay, to the outer face of an LCD as an interface between the display and the viewer. The overlay may be any suitable transparent material, including tempered glass or transparent plastic. Such an overlay may provide desired aesthetic features as well as functional features to the display. For example, some overlays may be used to create a smooth, transparent cover over the display, as in a cell phone, computer monitor or television. Further, some overlays may improve the robustness of a fragile LCD or OLED. The overlay may provide mechanical and/or environmental protection in displays which are stressed by their environments, including displays in public kiosks or ATMs, or in displays where a digitizer is used with a pen or stylus on the display and the overlay operates to protect the soft, polymeric top polarizer on the LCD or films and materials within the OLED. Moreover, the overlay may also be actively functional, providing a touch interface or EMI shielding.
Although the overlay improves the LCD, OLED or other display device, the addition of the overlay introduces two additional air-overlay interfaces. These air-overlay interfaces generate reflections and decrease the performance of the display in use. For example, typical optical glass and plastics have an index of refraction between 1.47 and 1.59, resulting in reflections in from 3.6% to 5.3% at normal incidence at each surface. This roughly 10% increase in reflectance from the display caused by the addition of the overlay may dramatically decrease the performance of the display in use. The degradation of performance of the display based on the additional air-overlay interfaces may be significantly more evident in bright ambient environments.
To reduce the impact of adding an overlay, prior systems have bonded the overlay to the display using an adhesive. The adhesive has been used to fill the gap between the bottom of the overlay and the top face of the display. Currently, there are two types of optical adhesive in use for bonding the display: form-in-place liquid adhesives and pressure sensitive adhesives (PSA) sheets.
Form-in-place liquid adhesives may be applied to either the front face of the display or the back surface of the overlay. The overlay or display is then positioned onto the adhesive while the adhesive is uncured. The display and overlay are held in place as the adhesive cures. In some cases, the adhesives are self curing. In other cases, UV light is used to initiate cure. However, the form-in-place liquid adhesives may have substantial limitations and may increase production costs of a completed display. For example, a long period of time is necessary to enable the form-in-place liquid adhesive to sufficiently cure. Further, accelerating curing may be difficult as the overlay and the display provide a barrier to any heat or light source used to accelerate curing.
PSA sheets include sheets with pressure sensitive adhesive where the adhesive forms a bond when pressure is applied. Although the PSA bonds may prevent some of the difficulties that arise in regards to the liquid form adhesives, the mechanical properties of the adhesive are driven by the roll-to-roll process that is used to create them. By necessity, the PSA sheets are harder and stiffer than desired for direct bonding applications. The requirements for using the PSA sheets result in direct transmission of force from the overlay to the fragile LCD surface during adhesive application, resulting in optical defects such as mura or even permanent cell damage. OLEDs are also susceptible to damage since the OLEDs typically incorporate a very thin glass substrate. Additionally, the increased surface hardness of the PSA systems increase susceptibility to latent processing defects when contaminants are present on the surface to be bonded. For example, the PSA will not conform to a surface contaminant and a delamination or bubble will form around the contaminant after bonding. PSA sheets are also limited to small thicknesses as they must be free standing. In many cases, the thicknesses which the PSA sheets can accommodate are insufficient to fill the gap between the top polarizer of an LCD and a bottom surface of an overlay. Most displays larger than 5.0″ diagonal include a bezel surrounding the display to provide structure and rigidity to the display system. This bezel fixes the minimum thickness of the optical adhesive between the top polarizer and the overlay and is typically 0.5 mm to as much as 3.0 mm for large displays. The PSA sheets cannot accommodate such adhesive depth requirements.
The inventors herein have recognized that there exists a need for providing improved viewing characteristics for displays, such as LCDs and OLEDs and a need to improve current methods for applying an overlay. Thus, as described in the disclosure below and as illustrated in the example figures, the inventors have provided methods, processes, systems and apparatus for providing an improved display with an overlay, including methods, processes, systems and apparatus for bonding an overlay on an LCD, OLED or other display device.